Gland Thread for Cylinders

1- Not all have or use the source you mention so that it would be better to explain what every symbol means 2- You could assume in a 1st good approximation that the full load is supported by 2...3 threads. You have to check their shear carrying capability. 3- You should also check how much the cylinders "grows" under pressure since this leads to a smaller shear area. Of course the most critical thread dimensions have to be considered (minimal gland thread dimensions and maximal cylinder thread dimensions)

Hi Samantha77,

An idea of the working pressure would help (3,000 psi will give you about 9.5 tons of thrust).

The rod looks about 1" diameter so the cylinder doesn't look particularly "heavy duty", the application and stroke length would also help ensure a suitable FOS (Factor of Safety) was included.

Is the thread 3 3/16" diameter by 12 Threads per Inch?.

Best regards

John

Thank you for the reply guys, below are the missing parameters for the cylinder.

• Bore: 3 in;
  
• Stroke: range of (18" ~ 24");
  
• Rod: 1.25" diameter;
  
• Operating pressure: ~2200psi.
  
• The thread on the gland is 3-3/16-12 and currently has a thread engagement length of 1.188"
  
• Force on Extend: 15,500 lbs approx;
  
• Force on Retract: 12,750 lbs approx

Below are the definitions for the letters in the formula:

Le= (2*At)/[3.1416*Knmax*(1/2+0.57735*n(Esmin-Knmax))]

• At= tensile-stress area of screw
• Knmax=maximum minor diameter of internal thread
• Esmin=minimum pitch diameter of external thread
• n=number of threads per inch
• Le= length of engagement (in inches)

I am trying to figure out the required length of the gland thread (currently 1.188" Lg but am getting weird results can someone guide me in the right direction please (website, books, formula etc)

Thank you all very much,

Samantha

The 1.188" of thread engagement works perfectly fine but I would like to reduce it if possible since I require a smaller envelope ( I have to reduce the width of piston/gland & Barrel , but I need to know what the minimum thread length engagement would be for the gland in-order to reduce it. Hope that answers your question.

Samantha

Hello Samantha,

I spoke to a cylinder manufacturer but the guy in sales didn't have a clue except to say there cylinders have 50mm of thread on the gland but work at 3000psi not much help I'm afraid.

Looking at the drawing of the gland, would it be possible, to move the piston rod bearing recess, closer to the front seal recess.

Then you could remove the material from the back of the gland, without touching the thread. If someone then advises you on the minimum thread length it will be an additional saving.

Best of luck,

John

Hello jesw55, I have also ask a cylinder manufacturer with no results. I actually did reduce the area where the rod bearing sits that saved me some, I just need to squeeze a little less than 3/8" more. Thank you for your assistance

In my experiance using threaded pipe, designed to contain possible explosions, the thread length is equal to the I.D. of the pipe in question.

Hi Sam,

I am quite surprised that nobody read your formula and noticed that it has NO mention of pressure or material strength. It is for a SCREW and MUST be adapted to your problem as follows:

- The force is generated by internal pressure in the volume closed by the gland The area is defined by the seals static and /or dynamic.

- The resistance is given by the resistance to SHEAR of the thread which depends on the materials used for the tube and for the gland itself, to be considered the WEAKEST

- As well the SMALLEST possible shear section MUST be for safety reasons considered for the computation thus the maximal internal thread position and the minimal external thread positions taken as references. Now the shear sections per thread (1 turn ) have to be computed one at the external thread top and one at the internal thread diameter. Each one is multiplied by the shear admissible stress for the respective materials( usually 1/3^0.5 of minimal yield strength of material, a safety coefficient has to be considered since there are dynamic pressure peaks or dynamic loads ). The SMALLEST value is accepted as maximal load / 1 thread and it is assumed that the pressure load is uniform over the threads since if one comes to the élasto-plastic range it will transfer load to the next and so on.

- The number of threads is obtained by division of total load by above portance which give the number of FULL threads and the length.

Make the computation and show results for further discussion or correction.

Do NOT forget to compute the increase of diameter in the thread region under internal pressure AND under axial force due to the thread flank results have to be ADDED to the geometrical dimensions coming from the dimensional standards.

I hope now it will help. In fact my first comment gave already all indications for the design.

I've been using this formula for the last 20 years on hydraulic cylinders. It does state that the calculated Le will be sufficient to break the 'bolt' At. So calculate At and Le and, for the material of the 'bolt', work out the load capability of At. I always used min. yield strength and 3 : 1 safety factor ( you can think of this as the Safe Working Load - SWL - of Le). Then use the simple ratio of: {[Max. hydraulic load on thread (usually full-bore load at max. working pressure)] ÷ SWL} x Le = Required Thread Length.

You have to be careful if gland material yield strength is higher than cylinder tube yield strength, because you then have to consider the 'J' factor - and that's another story.

Thank you Lftrsuk for that information, will do.

Samantha

Even if a formula was used for 20 years that not means it is the best.

As you may know technique presents a stady and continous evolution.